Currently, cold beverages such as juice and alcoholic beverages such as beer are sold in containers such as glass and PET bottles. Printed heat-shrinkable labels are often fitted to the outside of the containers in order to differentiate from other products and to improve product visibility. Normally polyvinyl chloride, polyester, polystyrene and the like are used as the materials for these heat-shrinkable labels.
Polyvinyl chloride group (called “PVC group” hereinafter) heat-shrinkable films have satisfactory finish of shrinkage and natural shrinkage characteristics (specifically, the natural shrinkage rate is small), and have been broadly used in the past for heat-shrinkable labels. Nonetheless, when incinerating after use, the PVC group produces toxic gases such as hydrogen chloride and dioxin, and therefore in recent years heat-shrinkable films using materials to substitute for the PVC group have been developed from the perspective of environmental safety.
In addition, although polystyrene group heat-shrinkable films, which have styrene-butadiene block copolymer (SBS) as the main material, have the advantage of better finish of shrinkage than PVC group and polyester group heat-shrinkable films, the polystyrene group has such problems as being less elasticity and having inferior natural shrinkage.
Polyester group heat-shrinkable films, which are rigid at room temperature, have low-temperature shrinkage, and have excellent natural shrinkage, are mainly used for the aforementioned uses. Nonetheless, compared to PVC group heat-shrinkable films, polyester group heat-shrinkable films have the problem of being prone to produce shrink marks and wrinkles.
Meanwhile, the aforementioned plastic films are chemically stable and do not decompose when discarded into the natural environment, and therefore there is the concern about the problems of accumulation as trash and the advance of environmental pollution. Moreover, the aforementioned plastic films are produced from petrochemical resources such as oil, and thus there is concern about exhausting petrochemical resources in the future.
From the perspective of reducing the aforementioned problems, biodegradable plastics derived from plants such as polylactic acid resins are known as materials that contribute to the economization of petrochemical resources.
These polylactic acid resins are plastics derived from plants such as corn and potatoes that are a source of lactic acid obtained from starches, and have gained particular attention for use in films and the like because of superior transparency.
However, because of the brittleness of the material itself, polylactic acid resin alone is not sufficiently strong when formed into sheets or films, and is difficult to use in practical applications. Specifically, with uniaxial shrinkable film that is drawn out along one axis, the brittleness in the direction that is not extended is not improved by extension, and adequate mechanical characteristics such as impact resistance cannot be obtained. In addition, crystallization is promoted when heating, and there is the problem that sufficient heat shrinkage cannot be obtained.
Various methods that have a resin composition contained in the polylactic acid resin have been proposed to improve such mechanical characteristics as the impact resistance of the aforementioned polylactic acid resin. For example, the following have been disclosed: a compound having polymethacrylic acrylate resin contained in polylactic acid resins have a specified weight mean molecular weight (refer to Patent Literature 1); a compound having aliphatic polyesters other than polylactic acid contained in the polylactic acid resin (refer to Patent Literature 2); a compound having polycaprolactone contained in the polylactic acid (refer to Patent Literature 3); a compound having polyolefins such as ethylene-vinyl acetate copolymer contained in the polylactic acid resin (refer to Patent Literature 4); a compound having aliphatic aromatic polyesters contained in polylactic acid resins with an adjusted L-lactic acid and D-lactic acid copolymer ratio (refer to Patent Literature 5); a compound having a polyolefin such as ethylene-vinyl acetate contained in the polylactic acid resins (refer to Patent Literature 6); and a compound with improved finish of shrinkage by adjusting the degree of crystallization of the polylactic acid resin, and by further blending an aliphatic polyester resin (refer to Patent Literature 7).
However, the main object of the polylactic acid resins described in the aforementioned Patent Literature 1 was to improve the heat resistance and transparency, and there is the problem of applying this to improving the finish of shrinkage as a heat-shrinkable film. The object of the polylactic acid resins described in Patent Literature 2 to 4 was to improve brittleness characteristics which maintaining transparency, and it is difficult to apply this to improving the finish of shrinkage as a heat-shrinkable film.
Further, the polylactic acid resin described in Patent Literature 5 and 7 could suppress the crystallization of the heat-shrinkable film when heating, but there are the problems of shrinkage shrink marks, wrinkles and blisters produced by sudden shrinkage. Further, the polylactic acid resin described in the aforementioned Patent Literature 6 has the problem that a satisfactory finish of shrinkage as a heat-shrinkable film could not yet be obtained compared to that of the polyvinyl chloride heat-shrinkable films.
Further, the following methods to improve the brittleness of the polylactic acid resins have been disclosed: a method using a composition comprising a compound of polylactic acid and modified olefin (refer to Patent Literature 8); a method using a plasticized polylactic acid composition comprising a polymer with polylactic acid as the main component, and an aliphatic polyester plasticizer comprising aliphatic carboxylic acid and chained molecular diols as the main components (refer to Patent Literature 9); a method using a biodegradable resin composition comprising polylactic acid and epoxyized diene block copolymers (refer to Patent Literature 10); a method using a lactic acid polymer composition comprising polylactic acid, aliphatic polyester, and polycaprolactone (refer to Patent Literature 11); and a method using a polylactic acid resin composition comprising crystalline polylactic acid and at least one rubber component selected from natural rubber and polyisoprene (refer to Patent Literature 12).
However, when mixing the aforementioned polycaprolactone, modified olefin compound, epoxyized diene group block copolymer, natural rubber and polyisoprene and the like into the lactic acid resin, although an effect to improve impact resistance is observed, there is a notable loss of transparency as a result, and it is difficult to say that, for example, the technology is sufficient for uses that require confirmation of the contents inside the packaging material.
Moreover, there is a well-known method to improve impact resistance by compounding into polylactic acid resin impact improvement agents with a multi-layer structure that contains polyacetal resin and diene rubber, natural rubber, silicone rubber, polyurethane rubber, of methyl (meth)acrylate in a shell layer, and at least one selected from a styrene unit and a butadiene unite in a core layer (refer to Patent Literature 13), but this is not sufficient as a heat-shrinkable film.
Further, a method compounding a graft copolymer by graft copolymerization of a rubber polymer and vinyl monomer in the polylactic acid resin has been proposed (refer to Patent Literature 13), but this is not sufficient as a heat-shrinkable film.    Patent Literature 1: Japan Laid-open Patent Application No. 2005-036054    Patent Literature 2: Japan Laid-open Patent Application No. H9-169896    Patent Literature 3: Japan Laid-open Patent Application No. H8-300481    Patent Literature 4: Japan Laid-open Patent Application No. H9-151310    Patent Literature 5: Japan Laid-open Patent Application No. 2003-119367    Patent Literature 6: Japan Laid-open Patent Application No. 2001-011214    Patent Literature 7: Japan Laid-open Patent Application No. 2000-280342    Patent Literature 8: Japan Laid-open Patent Application No. H09-316310    Patent Literature 9: Japan Laid-open Patent Application No. 2000-191895    Patent Literature 10: Japan Laid-open Patent Application No. 2000-219803    Patent Literature 11: Japan Laid-open Patent Application No. 2001-031853    Patent Literature 12: Japan Laid-open Patent Application No. 2003-183488    Patent Literature 13: Japan Laid-open Patent Application No. 2003-286400    Patent Literature 14: Japan Laid-open Patent Application No. 2004-285258